When an optical base material such as a glass lens or a plastic lens is directly used, reflection and glittering of the surface often harmfully affects the optical performance. Generally, the optical base material such as a glass lens or a plastic lens is surface treated so as to depress the amount of reflection and to improve the light-transmittance. One specific method for preventing reflection is a method of coating an inorganic oxide or halide by vacuum vapor deposition.
However, in such a reflection preventing film, contaminations due to sweat, dirt from hands, fingerprints, hair dressing, oils, etc. easily adhere to the film and are difficult to remove. Since such a film has a high wettability to water, if water or rain droplets adhere thereto, the water spreads widely across the film surface; thus, the refraction ratio of the optical parts is changed. Moreover, the water penetrates into the film, and elements and ions dissolved in the water cause chemical changes of the film itself which also helps to change the refraction ratio of the optical parts.
Generally, an SiO.sub.2 type or MgF.sub.2 type film is used as the reflection preventing film at the surface exposed to the air, i.e., the outermost coated layer. In the SiO.sub.2 type film, SiO.sub.2 reacts with water to form a silanol group, and a water mark due to this reaction is often generated. In the MgF.sub.2 type film, MgF.sub.2 reacts with water and Mg(OH).sub.2 deposits so as to cause a similar type of a water mark.
Furthermore, since the film surface of a vacuum vapor deposited film does not have good sliding properties, problems arise, e.g., the surface is easily damaged by abrasion.